catches

Sometimes you want to catch two different sorts of exception. You could do something like
> f = expr `catch` \ (ex :: ArithException) -> handleArith ex
> `catch` \ (ex :: IOException) -> handleIO ex
However, there are a couple of problems with this approach. The first is that having two exception handlers is inefficient. However, the more serious issue is that the second exception handler will catch exceptions in the first, e.g. in the example above, if handleArith throws an IOException then the second exception handler will catch it.
Instead, we provide a function catches, which would be used thus:
> f = expr `catches` [Handler (\ (ex :: ArithException) -> handleArith ex),
> Handler (\ (ex :: IOException) -> handleIO ex)]